Recently, attention focuses on a so-called white-color Light Emitting Device (LED) in which a yellow phosphor such as YAG:Ce is combined with a blue LED to emit white-color light by single chip. Conventionally, the LED emits red, green, or blue light in monochromatic form, and it is necessary that plural LEDs emitting monochrome wavelengths are driven in order to emit the white-color light or intermediate-color light. However, currently, the combination of the light emitting diode and the phosphor removes the trouble to obtain the white-color light with a simple structure.
An LED lamp in which the light emitting diode is used is applied to various display devices of a mobile device, a PC peripheral device, an OA device, various switches, a light source for backlight, and a display board. In the LED lamps, there is a strong demand for high efficiency. Additionally, there is a demand for high color rendering in general-purpose lighting applications, and there is a demand for high color gamut in LCD TV backlight applications. High efficiency of the phosphor is required for the purpose of the high efficiency of the LED lamp, and a white-color light source in which a phosphor emitting blue excitation light, a phosphor excited by blue light to emit green light, and a phosphor excited by blue light to emit red light are combined is preferable to the high color rendering and the high color gamut.
The high-power LED generates heat by drive, and generally the phosphor is heated up to about 100 to about 200° C. When the temperature rise is generated, generally emission intensity of the phosphor is degraded to generate so-called thermal quenching. Therefore, unfortunately the luminous efficiency is degraded particularly in a high-temperature range, that is, a high-current range.
Additionally, when plural phosphors are used, unfortunately the luminous efficiency is degraded by reabsorption between phosphors.